This might depend on how you measure the size of an image. If you right-click on any file in Windows Explorer & select Properties you will see two values,
Size:
&
Size on Disk:
I've noticed with images that these are different.
The file size on the forum upload page is measured in bytes which might also affect this. I can't find an image at exactly 100 Kb to check it out. I've posted literally hundreds of images on this forum without a problem. If the file is rejected for being too big I simply compress it a little more.
Note the indicated file size of Vp1400.jpg in this example.

This proves that uploading images does not affect the file size in any way.
